On DT you can sell the rights, which I think is what you are describing.  Alamy RM also could negotiate that kind of deal - that's the advantage of RM.  I'm sure there are others but probably not the usual micros.

yes, an affliate of mine who was at that time exclusive with dreamstime, was approached several times for her work. they also asked to see certain ones which i don't know how she went about doing it. do you know if the buyer is legit or not, was also the first thing she asked.
it is not uncommon, for a corporation to buy outright, for what? i think dreamstime default is like 300 dollars which is less than you pay a photographer for an assignment. and they already know what they get with a photograph from dreamstime costing 300, as opposed to hiring a local and not knowing if they are going to get what they want. 300 is cheap really to them, when you consider an actor pays 300 for headshots regularly . you can extrapolate and see why it is not too much to a corporate.  but i think too 300 is little, when you consider SStock earns me (and all) 85 and 105 dollars for an ext license respectively, and that does not involve transfer of ownership of that photograph.

so, price carefully when and if you get an offer to buy out. if that image is not earning you much, then it is as they say, money in the pocket today vs future potential of zero to dollars.
in most cases, i too would take the money today, then to wait till i grow a second beard on my lamb-chops hoping for that big dollar in the sky 

At DT you can set your price for Sell the Rights at anything you want. If you choose not to click this box (a wise move) and get a request from a buyer you can negotiate. There is a recommended default based on the popularity of the image but only uninformed contributors set it to the default.

that's good too, (cheers coffee)...
this way you set the price and let them talk you down to a reasonable amount that both of you find right.

Alamy RM also could negotiate that kind of deal - that's the advantage of RM. 

True, but there's no way you can 'shield' any special image from low value sales there. (You can opt out of the UK Newspaper scheme, and out of any or all distributor schemes, but some other sales are pretty low). Therefore you might have got $5 for a sale that then negates exclusivity, though they should have a record of the rights that were sold, which may be non-conflicting. Also from the horse's mouth, it happens very seldom.