The source said on Wednesday: "There are one or two technicalities that are out of our hands, so we have to wait and see. These may delay the signing of the deal until next week. The reports that it is a done deal are premature.
"But, having said that, we are very close to getting all the bank papers together. We're in the home straight but we can't see the finishing line yet. There's a 50/50 chance the deal will be signed on Friday but if not, then early next week."
Demolition of the old Wembley stadium will start as soon as the deal is finalised.
Building revealed in May that the construction cost of the stadium would be £410m, but sources have claimed the figure is rising as the delays drag on.
The stadium has been closed since England lost a World Cup qualifier to Germany in September 2000. However, the Football Association is planning to open the facility in time to host the 2006 FA Cup Final.
We are very close to getting all the bank papers together. We’re in the home straight, but we can’t see the finishing line yet
Senior source close to Wembley deal
WestLB and the FA have been in negotiations since April but have so far failed to sign a formal agreement.
The latest setback to the ground, popularly known as Bertie's Bowl, follows a decision by the Irish cabinet to withdraw public funding. It has now been advertised in national newspapers.
It is known that several sports management and construction firms have been in contact about the Abbotstown project.