Seems like you have a case of buyer's regret? Remember, "shit happens" even with landlords. Not everyone who owns a property in this town is a scumbag / slumlord.
1) Has the landlord signed the PA and do you have a copy of the signed agreement? If thats the case, the landlord cannot change the dates and they're as liable as you to complete on the 7 day mark - not earlier, not later.
2) If you do like the place, can you go back and tell the landlord and get them to sign the final agreement and you will sign on the date assigned and exchange contracts / payments and take possession on the date assigned in the provisional agreement. Payment can be made by cashiers cheque if needed to the agent or deposited directly to the landlord's account.
This would apply if the landlord has an emergency is going out of town and trust you and the agent to do the right thing.
Bottom line - if you've signed the PA, you cannot back out without paying your dues and neither can the landlord.