"'A marital settlement agreement is a contract between the parties. Where the agreement permits modifications, those modifications require a showing of a change in circumstances. Moreover, in determining what constitutes a change in circumstances the trial court is bound to give effect to the intent and reasonable expectations of the parties as expressed in the agreement,' and, thus, 'the trial court's discretion to modify the spousal support order is constrained by the terms of the marital settlement agreement.' (In re Marriage of Anninger (1990) 220 Cal.App.3d 230, 238.) In In re Marriage of Norvall (1987) 192 Cal.App.3d 1047, the appellate court held that evidence of income produced from a community property source awarded to the wife in a stipulated settlement agreement did not constitute substantial evidence of a material change of circumstances to justify a reduction in the husband's spousal support obligation." (In re Marriage of Dietz (2009) 176 Cal.App.4th 387, 398.) "Similarly, in In re Marriage of Rabkin (1986) 179 Cal.App.3d 1071, 1081, the appellate court reversed an order reducing the husband's spousal support obligation, stating in part: 'We have concluded that none of these factors furnished a proper basis for the trial court's $ 250 per month reduction in wife's permanent spousal support. As for the sale of the family residence and wife's resulting right to receive approximately $1,800 in monthly mortgage payments, such right constituted the single major asset awarded to wife as her one-half share of the community property. The parties' agreement, which was very carefully drafted by skilled attorneys, provided that wife would receive her share of the community property and spousal support. It makes no more sense to reduce wife's spousal support because she received her rightful share of the community property than it would to increase wife's spousal support because husband received his rightful share of the community property. . . . (In re Marriage of Dietz, supra, 176 Cal.App.4th at p. 399.)