Freewill offering/offerings is mentioned in the Bible.
Strong’s Definitions
נְדָבָה nᵉdâbâh, ned-aw-baw'; from H5068; properly (abstractly) spontaneity, or (adjectively) spontaneous; also (concretely) a spontaneous or (by inference, in plural) abundant gift:—free(-will) offering, freely, plentiful, voluntary(-ily, offering), willing(-ly), offering).
KJV Translation Count — Total: 26x
The KJV translates Strong's H5071 in the following manner: freewill offering (15x), offerings (9x), free offering (2x), freely (2x), willing offering (1x), voluntary offering (1x), plentiful (1x), voluntarily (1x), voluntary (1x), willing (1x), willingly (1x).
Outline of Biblical Usage
Brown-Driver-Briggs Lexicon
STRONGS H5071:
Abbreviations
† נְדָבָה noun feminine voluntariness, freewill-offering; — absolute נ׳ Exodus 35:29 14 times; construct נִדְבַת Deuteronomy 16:10; plural נְדָבוֺת Amos 4:5; Psalm 68:10; נְדָבֹת Psalm 110:3; construct נִדְבוֺת Psalm 119:108; 2 Chronicles 31:14; suffix נִדְבֹתֶ֫יךָ Deuteronomy 12:7 4 times suffixes; —
1. voluntariness אֹהֲבֵם נְדָבָה Hosea 14:5 I (י׳) will love them freely; עַמְּךָ נְדָבֹת בְּיוֺם חֵילֶ֑ךָ Psalm 110:3 thy people will be (all) voluntariness in the day of thy host (be ready to volunteer, see BrMP 133); גֶּשֶׁם נדבות Psalm 68:10 generous (copious) rain; בנדבה Psalm 54:8 voluntarily, and adverb. accusative נדבה Deuteronomy 23:24 (see Dr.).
2. freewill, voluntary, offering:
a. including gifts used in erection and furnishing of tabernacle Exodus 35:29; Exodus 36:3 (P); for first temple 2 Chronicles 31:14, second Ezra 1:6; Ezra 8:28.
b. including עולה and שׁלמים Ezekiel 46:42; probably also Amos 4:5, נדבות פִי Psalm 119:108.
c. usually a זֶבַח for festival meals at Passover 2 Chronicles 35:8, Pentecost Deuteronomy 16:10, Tabernacles Ezra 3:5 (besides the ritual offerings); distinguished from נֶדֶר votive offering Leviticus 7:16; Numbers 15:3; Numbers 29:39 (P), Leviticus 22:18,21,23; Leviticus 23:38 (H), Deuteronomy 12:6,17. — (Verbs used are הביא נ׳ Exodus 35:29; Exodus 36:3; הקריב נ׳ Leviticus 22:18; עשׂה נ׳ Numbers 15:3; Ezekiel 46:12; נתן נ׳ Leviticus 23:38; Deuteronomy 16:10.)

