Los Arboles Tulum, L.P. and Greg Schnurr v. Matula Family, L.P., 03-23-00488-CV

In 2012 Matula Family, L.P. loaned Los Arboles Tulum, L.P. $750,000 by promissory note; Greg Schnurr signed both the note (in a corporate-capacity block) and a separate personal guaranty.
An Amendment increased principal by $100,000 and stated the guaranty would “convey” to the Amendment; Schnurr signed the Amendment in the same block used for the note.
Los Arboles and Schnurr defaulted; Matula moved for partial summary judgment on breach of the note and guaranty and submitted a manager’s declaration and an amortization schedule reflecting compounded interest.
Schnurr opposed on the ground the Amendment was a material alteration that discharged his guaranty but did not challenge Matula’s interest calculation or assert simple-interest/usury before summary judgment.
The trial court granted summary judgment for Matula (judgment ~ $2.68M); Schnurr later filed amended answers asserting interest-based counterclaims that were stricken as untimely; his new-trial motion was overruled by operation of law.
Schnurr appealed, raising (1) material-alteration defense to the guaranty and (2) challenges to the interest calculation (compound v. simple, 360-day convention, early application of post-maturity rate).

Issue	Plaintiff's Argument (Matula)	Defendant's Argument (Schnurr)	Held
Material alteration of the note discharging guaranty	Amendment expressly carried the guaranty; Schnurr signed the Amendment and, as owner/manager, consented	Amendment materially altered the note and Schnurr’s signature on the Amendment was in his corporate capacity only, so he did not consent personally	Court: No fact issue—Schnurr, as owner/manager, consented; alteration made by him cannot be said made without his consent; summary judgment for Matula affirmed
Interest calculation (compound v. simple; 360‑day year; timing of post‑maturity rate)	Matula’s declaration and amortization schedule (unchallenged at summary judgment) correctly calculated interest	Note provides for simple interest; Matula used compound interest and improper day‑count and timing (arguments raised only after summary judgment)	Court: Schnurr waived these arguments by not raising them in his summary‑judgment response; counterclaims stricken; interest award stands

B.C. v. Steak N Shake Operations, Inc., 598 S.W.3d 256 (Tex. 2020) (recital that court considered non‑movant’s untimely response preserves issue)
Old Colony Ins. v. City of Quitman, 352 S.W.2d 452 (Tex. 1961) (material alteration doctrine protects guarantor from changes made without consent)
Chambers v. NCNB Tex. Nat’l Bank, 841 S.W.2d 132 (Tex. App.—Houston [14th Dist.] 1992) (alteration by guarantor himself is not an alteration made without guarantor’s consent)
Beal Bank, SSB v. Biggers, 227 S.W.3d 187 (Tex. App.—Houston [1st Dist.] 2007) (distinguishing modifications where guarantor signature line was left blank)
Unifund CCR Partners v. Weaver, 262 S.W.3d 796 (Tex. 2008) (issues first raised post‑judgment are waived)
Brumfield v. Williamson, 634 S.W.3d 170 (Tex. App.—Houston [1st Dist.] 2021) (trial court cannot permit amendment of pleadings after rendering summary judgment)