I have a square butt. It’s saggy skin close the the butt crack area. Don’t want a bigger butt, can this be fixed? (photos)

The appearance of and concerns you have with your buttock typically stem from deflation and/or aging of the buttock tissue. Therefore the best way to rejuvenate your buttock is to restore the volume loss first and foremost. Options include fat transfer or buttock implants.  Buttock implants, however, are not only safer but permanent whereas fat transfer commonly melts away and happens to be the most deadly cosmetic  surgical procedure available nowadays. Glad to help. 

This can be fixed by taking the unwanted fat from another part of your body like thighs, abdomen, flanks etc. This fat can be used to shape your buttock. With shaping your buttocks, I mean that we can give different shape to your buttock, not necessarily increasing the size. You may just increase its projection or its width, depending on what you want. In my practice, I show my patients 3-4 types of different buttocks and they can choose the shape they desire. Consult a board certified plastic surgeon who can examine you and guide you further.

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for fat transfer or butt implants to fix your problem, and I think you'll get very good results. 
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)