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Only SBD, the upvote is 180% of the payment, so the post value is increased by x 1.8 of the payment :)

ok.
OK.

I want to say something that bothers me. For example, I send 15 SBD for a vote and you give a vote using only 8 SBD and return me 7. With that, I lose part of my posting's growth with your account. Why does that happen?

It's simply a question of availability. Some bots won't even give a refund.