You are now asking a different question ...
if you said on the 2017 ACA application that you would make $20K and you don't then on the tax return you are not penalized for not making enough.... however you may fall in to the "hole" situation for the next tax year as described below ...
Now for the 2018 application ... if you indicate you will not make a minimum amount per the rules then you are not eligible for the advance credit (or ACA insurance) since you are not required to have insurance since you are below the poverty level. In state's that did not expand medicaid there is a weird "hole" that you can fall into ... you either make so little that you are medicaid eligible or you make enough to get the ACA benefits but if you fall in the middle (the "Hole") you are not poor enough for medicaid and not wealthy enough for the ACA.
If you don't understand this then talk to any health insurance person locally and they will be happy to educate you.
