Is there a way to reallocate the funds (on the same contract) prior to the final price revision to purchase additional items with in scope?
There are two tests to consider when trying to determine if a change is in scope. The first test is for the contemplation of contracting parties. Was there anything in the solicitation that would have indicated that the government might buy more systems? Were there any discussions held on options or additive bid items? If there was nothing in the solicitation or the negotiation record, it would be hard to say this was within the realm of what the contracting parties would have contemplated. But for argument's sake, let's say it was within the contemplation of the parties. In that case we need to move on to the second test.
In this test, we need to consider the scope of the competition. You look to have a $54 Million surplus and at $7 Million a copy that is 7 or maybe 8 more systems. Would that extra dollar value incented other contractors to propose? Would the field of competition changed?
Buying those items from this current contractor would be a sole source purchase. As such it normally would require a Justification and Approval for Other Than Full and Open Competition. Presuming you could get that J&A signed, then I think you could move forward.