Hi Monica,
The ETC Field (PRESTSUM) is a virtual field on the Task object, so it won't be able to be mapped at the Task level to MSP. It can be mapped at the assignment level though with PPM Attribute ID of prEstSum. (Or at the task level, ETC does export to MSP as the MSP field "Remaining Work".)
For the Save issue, are you using the MSP New Driver or Legacy driver? If the New Driver, do you get a Save Successful message still when saving back? Does it happen on multiple projects? What type of change are you making to the task? (And as Srikanth mentioned, if you get an error on the Save, if you can provide the error, that would be helpful too)
Thanks,
Kristin