(b) For any district that would have a reduction of 25 percent or more in its federal grant pursuant to Public Law 81-874 if the average daily attendance of pupils residing within the district were credited to the district of attendance, then the average daily attendance resulting from an interdistrict attendance agreement shall be credited to the district of residence and the district of residence shall pay a tuition to the district of attendance no later than the next August 31, after the close of the fiscal year as follows:

1. (1) For attendance in regular day schools and summer schools the tuition per unit of average daily attendance, if any, shall not exceed the actual cost per unit of average daily attendance for the grade level or program less any income, other than tuition, received by the district of attendance on account of such attendance.

2. (2) The district in which the pupil resides shall reduce its total revenue limit pursuant to Section 42238 by the total excess, if any, of its revenue limit per unit of average daily attendance multiplied by the total interdistrict attendance over the total tuition to be paid to districts of attendance.

   The district in which the pupil resides may increase its total revenue limit pursuant to Section 42238 by the total excess, if any, of the total tuition to be paid to districts of attendance over the district of residence’s revenue limit per unit of average daily attendance multiplied by the total interdistrict average daily attendance.