Any Safe Temporary Fillers for the Butt?

all of the previoius answers are correct.   fat is the best filler.  all the others are not fda approved and would require huge amounts.

sagging can be improved with a surgical lift and the incision may be a worthwhile trade-off for the shape that you obtain

There are no FDA approved fillers for buttock augmentation. There are some fillers that are used for the face which could be used "off-label" for the buttock but will require numerous syringes and only give a temporary effect. The best material is your body's own fat.

If you communicate with your plastic surgeon your aesthetic goals with regard to the shape and size you want, the two of you should be able to agree on a reasonable surgical plan. This depends on the amount of fatty tissue you have of course.

A board certified plastic surgeon can examine you and determine if you are a good candidate.

Best wishes,

Dr.Bruno