Qitzur Shulchan Arukh – 65:23
It is permissible to pay an increase on a dowry. For example: He promised a dowry for his daughter, and made an agreement with his [future] son-in-law that every year he [the son-in-law] leaves the dowry with him [the father], he will pay him such-and-such profit, this is allowed. Because this is just like adding on more dowry to him, and as as if he said to him, ”I will give you a present of such and such on a certain date, and if I don’t give it to you on this date, I will add on for you such and such.” This is allowed. This is specifically if they agreed on this at the time of writing the [pre-marital] terms of agreement, since until now he didn’t pledge anything, all of it is now treated as one obligation.
However, if at the time of writing the terms of agreement, he obligated himself without conditions such-and-such as dowry, and at the time of the wedding he wants to change it, and give him a certain amount in return for extending the time, this is forbidden, and he can do it through a permissible loophole [such as the heter iska discussed in ch. 66.].