Under NEC Contract ECC can upfront material payments be retracted later under Clause 50.5?

Supposing that the Contract is ECC Option B and a group of BOQ items is for supply of material. The Contractor should only submit an application for payment on these items once the work is done, i.e. they have been delivered. However, as verbally agreed with the PM and to assist the Contractor with cashflow, the Contractor submitted an application for payment for these on the date of him ordering the material and this was assessed and paid by the PM.

Can the PM change his mind a few months / years later and decide that the material payment certificate was wrongly assessed (using 50.5) and then backcharge the Contractor interest for the upfront payment on the materials?

JKR - If there was in fact an agreement then the PM should not (and arguably cannot) go back on it. The real question is whether there was an agreement.

If there was an agreement then probably PM cannot change his mind.

If there was not an agreement then the usual application of 50.5 would allow, and in fact require, the earlier payment to be corrected. However, if there has been an attempt to mislead or inappropriately procure a profit (because the interest rate in the contract is perhaps well above that which is commercially available) then I suspect any tribunal would work hard to prevent the interest from being recoverable.

This is a very fact sensitive question without depth of facts the answer is just that 50.5 and 51.3 require a correcting amount and for interest to be paid.

Interesting if in Main Option A and B the contractor is only paid for Plant and Materials incorporated in the works (unless stated otherwise and P&M materials after being inspected by the Supervisor clause 7 and marked for the project and usually a vesting certificate and insurances) - does the PM have the authority to make such payments? I take a rather simplistic view that the PM adminsters the contract he has been given?