Similar expressions, but they have a slightly different meaning.
“On behalf of” someone = in the name of someone (representing someone)
Don’t worry on my behalf. (instead of me)
I am proud to accept this award on behalf of Georgina who is unable to be with us today. (that means ..I accepted an award for her because she could not make it to the ceremony but I”ll give it to her)
“In behalf of” someone = to favour someone (to benefit /for the benefit of /in order to help someone)
We raised money in behalf of the orphans. (for the orphans)
See both in the same sentence.
They are collecting on behalf of (representing) the charity which is seeking funds in behalf of (to favour) its cause.
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