Why not?
Assuming AA is being honest when he says he wants to get his own look at all these prospects BEFORE deciding who to trade (or not to trade), why would you back yourself into the corner that you can't afford the player you would consider trading for?
We're fans - of course we want him to run out and get players to fill holes before camp starts.
If we're going to entertain the blackjack dealer's wet dream of going after Donaldson and that the Jays somehow can't work out an extension with him before the deadline, somebody's going to have to pay the $11.5 million he's due during the second half of 2018. If AA thinks he's actually got a legitimate shot at extending him and could get him at the deadline for a reasonable prospect cost, every dime they save now would could come in handy, no?
I don't think anyone realistically expects that to happen, but it sure would suck to know that Donaldson's actually willing to sign here long-term without testing the market and that you couldn't trade for him because you couldn't afford to pay him for 3 months this summer because you spent the money on Frazier or Nunez wouldn't it?
Keeping money held back in a one off situation seems pretty far fetched. If the Braves really wanted to contend this year they would maximize their payroll at the start of the year. That makes the most logical sense.