Splitting interest in property

Apologies if this has been asked before, couldn't find anything about it. May be slightly complicated and I will be seeing a solicitor but trying to get a rough idea now for tax implications.

A couple (A and B, not married) are taking out a joint mortgage to purchase a house. The house is currently owned by the parents of B, and it's being sold for below market value. Ignoring any tax issues, is there some kind of legal document which can be drawn up before completion to state that parents are gifting B x amount of an interest in the property.

Example if property is worth €500k, and being sold to the couple at €400k. Is it possible to say that the €100k difference is a gift to B only and that when the mortgage is paid off, B will own €300k and A will own €200k? In other words A is not technically benefiting from the gift from the parents to B.