A debtor to keep the whole law, or a believer freely fulfilling the righteous requirements of the law by faith not for reward, rewarded.

Ga5.3  a debtor to keep the whole law

• If we do good, picking a commandment to obey, in order to receive merit/ favor/ reward, we should rather/ are obligated to rather be/ a "debtor to keep the whole law". The alternative is tantamount to self starting a new cherry picking part-law religion. This is idolatry.
• But, if a we freely do good/ fulfill any command, NOT to receive merit/ favor/ reward, but freely, without obligation, (ie out of favor/ grace already received), with no compulsion, God in truth and in justice will reward us.  This is fulfilling the spirit of the law by grace. 

The motive is everything.