Can a lender veto the sale of a mortgaged (PPR or investment) property? I wasn't aware of that but I suppose given their interest in it it might not be that surprising...
Having said that it would appear that the OP has agreed a sale on the property. In the current climate it appears that the 15K shortfall is not a bad result. No harm in approaching the Financial Institution and looking for a deal with them on this basis. I.e. If you could come up with a further 7/8K in cash they are likely to accept this.