Yes, but there are some subtleties.
Page 12 of the Europe `40 2e rules says:
You must collect income if you can, but **_all other parts of the turn sequence are voluntary.
Thus, you are allowed to buy units at the start of the turn but skip completely your ability to place units at the end of the turn.
However, when describing the rules for placing units, page 23 says:
Move the newly purchased units from the mobilization zone on the game board to eligible spaces you have controlled since the start of your turn.
So, if you decide you are going to place any unit at all, it is an all-or-nothing approach. You couldn’t decide to place half your units and then save the other half for later.
Later on, the rules say that if you purchased more units than you have the ability to place, you have to sell the excess back and collect a refund. So I suspect that the designers’ preference would be for you to only save money, never units, but the rules as written do allow you to save your unplaced units._**