In principle, you should be able to place the bounty. For two reasons.
This is what the help center says (my emphasis):
Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on).
You offered only 150 and thus 300 should suffice in this case. But, actually, there is a missing detail there.
This is what the relevant MSE FAQ says (my emphasis):
You can offer as many bounties on a question as you want. However, only one bounty can be active on a question at a time. Moreover, any user may have at most 3 concurrent bounties at a time.
Note that if you offer several bounties on the same question, you will have to double the amount each time (or more). That is, if your first bounty was worth 50 reputation, your second bounty on the same question will have to be for at least 100, your third for at least 200 and so on. If you've already offered a bounty for more than 250, you can still offer more bounties for 500 (the maximum amount) as long as you like (or as long as you have the rep). This doubling applies only to bounties by the same user on the same question.
Thus, offering a new 500 should be possible, regardless of earlier bounties on the question.
If you had tried and it failed it'd seem to be a bug.