delivered the Opinion of the Court.
Thomas Sabo appeals from a December 31, 1985, judgment of the Second Judicial District Court, Silver Bow County. The judgment ordered Thomas to pay Stephanie $7,750 in past child support, pay her $3,904.85 from the sale of marital property, and pay her $2,201.23 in attorney fees plus 10 percent accruing interest.
We reverse in part and affirm in part.
Thomas Sabo raises two issues for our review:
1. Is a non-custodial father, who assumes permanent custody, and support of the children with the consent of the custodial mother, relieved of the obligation , to pay child support accruing after and during the consensual assumption of custody?
2. Did substantial evidence support the court’s finding that Stephanie was a partner with Thomas in SACO Investments?
After sixteen years of marriage, the Sabos were divorced on November 15, 1976, in Silver Bow County. At the time, Thomas was an attorney and Stephanie was a real estate agent. The divorce decree incorporated a settlement agreement in which Thomas agreed to pay Stephanie $125 per month per child, in support for their three minor children. Thomas’ support obligation for his son, Mark, ended in September 1979, when Mark turned eighteen.
Through February 1981, Thomas paid Stephanie $8,375 in child support. In March 1981, Stephanie filed a Uniform Reciprocal Enforcement of Support Act (URESA) claim against Thomas while she was living in Texas. Thomas paid $1,000 on the claim but paid no child support after that date.
In the summer of 1981, both the youngest daughter, Erin, and the youngest son, Matthew, began living permanently with Thomas in Bozeman. Thomas never brought any action to modify the children’s support payments to reflect the change in circumstances. While the children lived with Thomas, Stephanie had weekly dinners with them but made no claim for child support. Matthew turned eighteen in April 1982. Stephanie brought the present action on January 25, 1985, less than one month before Erin turned eighteen.
Issue No. 1
Is a non-custodial father, who assumes permanent custody and support of the children with the consent of the custodial mother, *254 relieved of the obligation to pay child support accruing after and during the consensual assumption of custody?
Generally, when child support becomes due under a dissolution decree, the support becomes a judgment debt similar to any other judgment for money. Section 40-4-208, MCA.
In Re Marriage of Carlson
(Mont. 1984), [
In
State of Washington ex rel. Blakeslee v. Horton
(Mont. 1986), [
“The law is clear that the arrearage in child support payment cannot be modified by the court upon any retroactive basis . . . These legal principles, however valid they may be as a general rule, are rendered impotent when the parties mutually agree that they be ignored and also carry out such agreement in actual fact . . . Equity cannot allow the mother to participate in nullification of the purpose of the law in fact and, at the same time, allow her to claim the benefit of it in theory, simply because there is a meter running which can total a dollar loss in child support. [Emphasis added.]” Blakeslee,722 P.2d at 1050-1051 .
Thomas Sabo accepted the children into his home and raised the children without any support from Stephanie, even though she was employed as a real estate agent after her return to Bozeman. Stephanie never asserted that her current request was founded upon any actual need of her children. Furthermore, she never suggested any implied need which had not been satisfied over the years, or
*255
which had now arisen and could possibly serve to validate her action at this belated point. See
Blakeslee,
Although Thomas took no action to change the terms of the divorce decree, he accepted sole responsibility for the children’s health, welfare and support. During the period of Thomas’ care, Stephanie’s major contribution was sharing her Mexican vacation with Erin. Such inequity cannot validate her claim. “Although legally the mother may have been correct in her claim for child support, equity demands that the claim must fail. This Court has long adhered to such principles of equity.”
Blakeslee,
The Sabo children'voluntarily moved in with Thomas. During the entire period the children lived with Thomas, Stephanie neither pursued support payments in arrears nor objected to the shift in custody. By her assent and conduct, Stephanie consented to the arrangement. As we recently held, “[T]he equitable principle arises when the mother has expressly or impliedly consented to an arrangement other than the payment of the judgment.”
In Re Marriage of Cook
(Mont. 1986), [
On the issue of child support in arrears, we agreed with the
Cook
district court that the parents had entered into a binding oral agreement modifying support and visitation. The father relied on the agreement to his financial detriment. We concluded that the mother was estopped from enforcing the support provisions of the decree after the date they entered into the oral agreement. “Furthermore, the changes in the children’s residences represented changed circumstances so substantial and continuing as to make enforcement of (the father’s) original and modified support obligation unconscionable.”
Cook,
Never, during the entire period that the Sabo children were being raised by Thomas, did Stephanie offer any financial assistance to Thomas. In
Cook,
we noted that circumstances may not allow the mother to collect child support in arrears, “where the husband has made expenditures which constitute substantial compliance with the spirit and intent of the decree.”
Cook,
Stephanie consented to the change in custody and support. The question now becomes whether consensual modification of child support is enforceable in Montana and the extent of any such enforcement.
Upon compelling evidence, the doctrine of equitable estoppel may override the provisions of Section 40-4-208, MCA. This principle was best articulated in
In Re the Marriage of Jensen
(Mont. 1986), [
“We therefore hold that in Montana a decree for support may be modified on equitable grounds by a court where there is clear and compelling evidence of the terms of an oral agreement or modification. We further hold such modification may be applied only to maintenance and support payments to be made subsequent to the oral agreement for modification.”
These conclusions “are consistent with both
Blakeslee
and
Cook
where we enforced oral agreements pertaining to installments of support subsequently accruing.”
Jensen,
Under the equitable principles of
Blakeslee, Cook
and
Jensen,
we hold that Thomas substantially complied with the decree obligations. By her assent and conduct, Stephanie consented to the shift in custody and support. We cannot equitably allow Stephanie to reap a windfall of support payments, if she never made the support expenditures. “A party to an agreement which has been performed for some length of time is estopped to deny its validity.”
Jensen,
On Issue No. 1, we reverse.
Issue No. 2
Did substantial evidence support the court’s finding that Stephanie was a partner with Thomas in SACO Investments?
Thomas engaged in business activities with a realtor, Gene Cook, in SACO Investments. SACO’s primary purpose was investment in real estate. During the marriage, SACO bought some parcels known as the Hoff Property. In the settlement agreement, Thomas’ *257 share of the parcels was identified as the jointly-held property of Thomas and Stephanie.
After the divorce, SACO sold the one remaining parcel of the Hoff Property for $14,000. However, Thomas did not tell Stephanie about the sale or give her any money from the sale. Stephanie later learned of the sale from Cook. Thomas contended that Stephanie was not a SACO partner and, therefore, not entitled to any proceeds. However, the District Court found that she was a partner and entitled to one-half of Thomas’ share. Therefore, the court awarded her one-fourth of the total proceeds plus interest, or $3,904.85.
The court’s finding was well supported. Various SACO deeds identify Stephanie as a partner. The deeds were prepared by Thomas as attorney for the partnership. Furthermore, in his original response to Stephanie’s divorce filing, Thomas signed a sworn statement that the only assets he held, personally and apart from the marital estate, were his car and law practice. In addition, Thomas signed the property settlement agreement, listing SACO properties as “jointly held properties.”
These documents, signed by Thomas, are substantial evidence that Stephanie was entitled to participate in the partnership property. We will not set aside the findings of the District Court unless there is a clear abuse of discretion.
In Re the Marriage of Perry
(Mont. 1985), [
On Issue No. 2, we affirm.
