There really isn't anything build into Zen Cart to manage unit sales that are not consistant with Minimum, Unit and Maximum to control the quantities ...
You would really need to customize something for this to handle quantities where there isn't a clear pattern that can be tested for to manage this type of ordering ...
You could use Attributes for the pricing of these units as order 1 unit of 2500 ... but that is not accounting for the sale of quantity 2500 of the product but would show as a sale of 1 with a price at the 2500 unit rate ... and I don't know if that would accomplish what you are wanting, unless you are not concerned with the actual stock management ...



