This response is based on the information provided.  We suggest you discuss with your financial group/resource-funds manager, contracting team, program manager and/or legal department as appropriate. 

You identify a FFP - reimbursement CLIN on your contract, a contract type we have never heard of; those terms are mutually exclusive.  It would seem the arrangement could be that the contractor is reimbursed when it orders spares up to a certain dollar limit established on the CLIN.

As for the source of funding for the spares, it matters whether they are initial spares or replenishment spares.  If that this is true, then the funding required should come from funds available at the time the change is made