A — 1: φείδομαι
(Strong's #5339 — Verb — pheidomai — fi'-dom-ahee )

"to spare," i.e., "to forego" the infliction of that evil or retribution which was designed, is used with a negative in Acts 20:29; Romans 8:32; 11:21 (twice); 2—Corinthians 13:2; 2—Peter 2:4,5; positively, in 1—Corinthians 7:28; 2—Corinthians 1:3; rendered "forbear" in 2—Corinthians 12:6 . See FORBEAR.

Luke 15:17
B — 1: φειδομένως
(Strong's #5340 — Adverb — pheidomenos — fi-dom-en'-oce )

akin to A, "sparingly," occurs in 2—Corinthians 9:6 (twice), of sowing and reaping.