Here are the current terms and conditions, copied from the amex web site:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">You may purchase points in 1,000-point increments for $25 per 1,000 points, which will be billed on your Card account. You may only purchase a maximum of 500,000 points per calendar year. You may purchase points only for your own Program account. Points may not be purchased except in connection with a conversion of points to a participating Frequent Customer Program or redemption of points for a Reward.</font>
Interestingly, it appears that the purchase limit has been changed from 20K per month to 500K per calendar year. So, it appears that if you wanted to spend $12,500, you could purchase 500,000 miles in one transaction on your favorite MR-participating airline.
The "in connection with a conversion" provision means that you must immediately transfer the purchased MR points to a partner program, or otherwise redeem a MR award. That is, you can't just purchase extra MR points and have them sit in your MR account.
But, there doesn't appear to be any restriction relating to your current balance when purchasing points, such as a requirement that your transfer not otherwise be doable without the points purchase. To put it another way, there doesn't appear to be anything to prevent someone with a 100K MR point balance from purchasing 20K of points and transferring all of them to their favorite MR airline, just because they could also accomplish this by using some of their banked points.
Also, even if the term "in connection with a conversion" were to be interpreted as requiring some existing points to be transferred as part of the transaction, the above could be accomplished by purchasing 19K points and coverting 1K, for a total of a 20K transfer.
There's just no way the written terms of the program could resonably be interpreted as having anything to do with the amount of the purchase as compared to your current point balance. Of course, that's not to say that some MR rep hasn't said this was so at some point. In my experience, these people aren't very well informed, and tend to just make stuff up when they're not sure. You may have to call back, or escalate the issue to someone that can understand the written word, but you should be able to get a purchase done without respect to your current balance.