SLAST in the Kinetis eDMA engine is a 32-bit number. As such it can be treated as either signed or unsigned. One might use this to implement a circular buffer in which case it makes sense to enter it as a negative number. The Init_eDMA bean doesn't allow this (called Address adjustment field in data source/destination).
Also - while not a bug a shortcoming is that while you can put an #include in the 'external object declaration' and use symbols in the Address field, you can not in the Address offset/adjustment fields. It would be nice to allow that (toggle D, H, or text), to be able to use #defines and formulas from stuff in the #include you can put in.