Services not downgraded

in the beginning of june I went to Telkom to cancel my contract. They told me a consultant will call me to assist with the cancelation. The consultant called me and informed me that cancelling the contract will incur cancelation cost, I should rather downgrade my package. I downgraded my package from 80 Gig to 5 gig. this was supposed to be effective from 1 July 2020. but today I get an invoice for R499, showing that Telkom charged me for 80 Gig for July. Why Telkom?