financial agreement can it be nullified?

Hi, I am getting maried.
My fiancee has a son with another woman both of them are working.
They had a son, a 5 year old out of wedlock too..
they are already 2 yrs separated/parted ways though the guy is still visiting his son on weekends.
The girl badly wanted my fiancee back but the guy won't go back to her he doesn't love her (before we met they already have a son, they were already separated as college fling when they met again and the girl "PIKOT" the guy, he stayed with the girl on and off before just to be visible to the child)
Because of bitterness the girl sued my fiancee,first on nullification of his paternal right (THEY are NOT married, the son carried his surname), then she decided to ask for council at PAO and was advised for RA 9262 Financial support coz she made excuses and lies that the guy don't visit his son and dont give money when in fact his giving her money. without further trouble my fiancee agreed to legally support his son, even though the girl really wants him to be imprisoned. It's all in bad blood.

The thing is we have a new born baby now. When we get married can we do something to nullify the agreement? the girl is really demanding about the money. How about our own children??Surprised

please give me light. It will be greatly appreciated.

Yes you can nullify the agreement when your hubby can prove that he is not the father by presenting the d.n.a. result in court.