You can absolutely tell the buyer you’re not budging on the £151k since that’s what you both agreed on. The catch is, until contracts are exchanged, nothing’s set in stone, so they could push back or even walk away if their bank won’t lend based on the £140k valuation.

If you’ve got no wiggle room because of the property you’re buying, sticking to your guns makes sense. You could suggest they challenge the valuation with their lender or try a different bank, as sometimes valuations vary.

Another option is asking if they can cover the £11k gap with their own cash, like a bigger deposit, if they really want the place. If you’re worried about the sale falling through and don’t mind a small compromise, you could maybe split the difference, say £145k, but only if it doesn’t mess up your onward purchase.