[Help! How to deal with a buyer cancellation request]

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I delivered a very long and big work, and the buyer requested cancellation because he says it’s not a good quality. 
I reached out and tried to suggest a review to see what can be improved but he said no and said ‘to be honest I already did most of it myself’. 
the cancellation was maybe 4 hours after the delivery and I’ve been working on it for over 2 weeks so there’s no way he did it in 4 hours. 
I feel tricked and used, I don’t know if he really did it or he took my work and just don’t want to pay me. 
it’s not fair, specially because I contacted Fiverr support over 24 hours ago and still didn’t get a reply. 
Help, time is running out and I’m not sure what to do, if I agree to the cancellation I will lose a large agony of money, time, effort and maybe it will affect my statistics. 
if I don’t agree - the buyer might leave me a bad review that will affect my statistics too which is not fair. 
please advice. 

[Question about a buyer asking for refund , with all requirements satisfied]

I just had a similar experience. 
I delivered a very long and big work, and the buyer requested cancellation because he says it’s not a good quality. 
I reached out and tried to suggest a review to see what can be improved but he said no and said ‘to be honest I already did most of it myself’. 
the cancellation was maybe 4 hours after the delivery and I’ve been working on it for over 2 weeks so there’s no way he did it in 4 hours. 
I feel tricked and used, I don’t know if he really did it or he took my work and just don’t want to pay me. 
it’s not fair, specially because I contacted Fiverr support over 24 hours ago and still didn’t get a reply.