Short answer. It depends on your property regime. Family Code Article 84 caps donations in marriage settlements at one-fifth of present property, but only where future spouses agree on a regime other than absolute community — anything beyond that fifth is void. Donations of future property follow the rules on wills instead.
What the law says
If the future spouses agree upon a regime other than the absolute community of property, they cannot donate to each other in their marriage settlements more than one-fifth of their present property. Any excess shall be considered void.
Family Code, Article 84 — Limit on Donations Between Future Spouses; Future Property. Read the full provision →
What the law says
Donations of future property shall be governed by the provisions on testamentary succession and the formalities of wills.
Family Code, Article 84 — Limit on Donations Between Future Spouses; Future Property. Read the full provision →
The one-fifth cap, and when it applies
Article 84 states: if the future spouses agree upon a regime other than the absolute community of property, they cannot donate to each other in their marriage settlements more than one-fifth of their present property. The cap is conditional, not universal — it applies specifically when you and your fiance choose a property regime other than absolute community, such as complete separation of property or conjugal partnership of gains, and put the donation in your marriage settlements.
What happens if the gift goes over the limit
The consequence is direct: any excess shall be considered void. A donation in the marriage settlements that exceeds one-fifth of the giver's present property is not simply reduced or renegotiated by this article — the portion beyond the fifth has no legal effect at all. Only the part within the one-fifth limit stands as a valid donation under this provision.
Present property versus property you do not have yet
The one-fifth cap is about present property — what your fiance already owns at the time. Property they do not yet have is treated differently: donations of future property shall be governed by the provisions on testamentary succession and the formalities of wills. So a promise to give you something your fiance expects to acquire later is not measured against the one-fifth limit at all; it is governed by the separate body of rules that apply to gifts made through a will.
What to check before assuming a figure
Work out first which property regime you and your fiance are actually adopting, since the one-fifth cap does not apply at all if you are going with absolute community. If you are under a different regime, confirm what counts as your fiance's present property and get a real figure for it, since one-fifth is measured against that, not against the value of the gift itself. Have any marriage settlement reviewed by a lawyer before signing it.