A person or organization that pays for or covers the cost of something.
From defray + -er suffix (one who does something). The -er suffix is extremely productive in English, creating agent nouns from verbs since Old English times.
Defrayer is a specialized business term for 'the one paying the bill'—it appears in contracts and formal agreements but rarely in everyday conversation because 'payer' is shorter and more common.
Complete word intelligence in one call. Free tier — 50 lookups/day.