Will changing a land boundaries affect my mortgage?

We currently in the process of buying a house with the help of a relative.

They wish to purchase a piece of land from the bottom of the garden that we want to buy, the money that they are will to pay us for it will give us the deposit we need to buy the house.

Understandably they wish to have a legal document saying that we will gift them the land at some point in the future.

The solicitor have been very unhelpful and have only just stated that this might be a problem a week before completion.

So what we wish to know is what can we do this?
Can we gift this land after the mortgage and house sale is complete without effecting the mortgage?
Any suggestion would be very helpful, as the way it's currently going I could be down £2000 and have no house to show for it.

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