元がん金きん

元がん本ぽん sense 1
元金 and 元本 both mean 'principal', but 元本 is the stricter, more formal financial/legal term while 元金 is slightly broader and can also mean seed money.
資し本ほんN3sense 2
資本 refers to capital used to run a business in general, whereas 元金 specifically denotes the original sum before interest or profit.
資し金きんN2sense 2
資金 is a general word for funds or money set aside for a purpose, while 元金 specifically means the original principal amount.
原げん資し
元金 is the everyday word for the principal in a loan or savings account, while 原資 is a more formal term used in business and investment contexts.

Opposites

利り息そくN2sense 1
元金 is the original sum lent or invested, while 利息 is the extra money earned or paid on top of it.
利り子しN2
元金 is the principal sum borrowed or invested, while 利子 is the extra amount paid on top of that principal.

現げん金きんN4sense 1
元金 (がんきん, 'principal') and 現金 (げんきん, 'cash') sound similar but refer to completely different financial concepts.
利り払ばらい
元金 is the principal amount borrowed, while 利払い is the separate payment made on the interest owed on that principal.
利り息そくN2
利息 is the extra amount charged or earned on money, while 元金 is the original principal sum before interest is added.

元金Usage100%
- がんきん
  Standard kanji form; the reading がんきん is the most common.
- もときん
  Alternate reading; less common but still used, especially in spoken or informal contexts.
もときん

Compound of 元 (origin, base) and 金 (money). The reading がんきん is the on'yomi, while もときん uses the kun'yomi for 元.