On the one hand, FedEx is trying to reduce what they might view as unnecessary expenditures. So that militates in favor of the rumour being true.
However, FedEx is going gangbusters in the US; it's the international business that's the problem. The 767s are designed to replace existing domestic aircraft. They do do some South America runs and others, but they are primarily in the US. Which is growing. And the new aircraft increase reliability and lower maintenance expense and in some cases fuel expense. So they are are an expenditure that, well not strictly -speaking utterly necessary, do have positive and quantifiable benefits for the operation. So all that militates against the rumor being true. Also, as pointed out, there would seem to be some potential candidates for those aircraft that would pay more for them then would Amazon.