Trying to sell

Hi,
My wife and I are trying to sell a house we lived in prior to our current house and the mortgage term ended in August, RBS are in the process of extending the term and as such have valued the house at circa £297K.
We have tried selling over 18 months, one off of £272K fell through.
The thing is we looked at letting then decided against it, the day we took it off the letting market someone went to view to let, but the estate agent told them it was no linger possible.
The family fell in love with our house and proposed renting with a view to save sufficient for a deposit to purchase. We like the couple and decided to rent (start next month).
Any way, because we want to sell we have agreed a selling price of £245K if they can get the deposit within 9-12 months.
A few years ago my brother bought his house for less than it was valued and the difference was given as a vendors gift, this seems not to be available now???
I was just wondering if anyone knows of any mortgage lenders that would lend against the value of the house rather than the sale price of the house allowing a better LTV for the purchaser?
Thanks for any help given.
Glen.