If she cannot afford a sheep, she must then take two turtledoves or two young pigeons
This just occurred to me: How many JWs find that going to conventions or going Door-to-Door™ or Meetings™ take away from opportunities to build a better life/future for themselves? How many would love to get a better education so they can provide for their families more easily? How many have no retirement savings?
With this scripture in mind, it seems to be acceptable (to Jehovah™, at least) to give Him™ an item of lower value in presenting your sacrifice if you "cannot afford a sheep". This means you have a choice in how much of an offering you give. The ability to "afford" something is subjective. It means that you can have priorities other than whatever you intend to Sacrifice™ to Jehovah™.
Do you need to work extra shifts so you can save for your vacation or education? Does it mean you have to skip a few Meetings™ to do so?
Do you want to put away some money from each pay to save for your retirement? Does it mean there is less to put in the Contribution Box™?
The WTS wants you to think that you shouldn't have priorities other than Preaching™ Door-to-Door™, going to Meetings™ and Conventions™, and putting your hard earned cash in the Contribution Box™. But it's all a choice. You can do/give what you can "afford", once all your other priorities are met.