finance clause

Just wondering what is the shortest contract time or subject to finance sale? I have just received a contract with 28 days + 28 days. Apparently the purchaser is pre approved. Should I negotiate this?

Hi, I'm in the middle of negotiations for a property that is being sold by a divorcing couple. It's been on the market for a few months now, I first put in an offer on the 5th November and I think we have now come to a verbal agreement on price (the wife was holding out for more money)...