Britain has made payments to Rwanda under the £120 million asylum scheme, and Kigali has started spending the money despite the policy being grounded by legal challenges.
The Rwandan government said it had begun spending the cash to be ready in time for last week’s flight, which was cancelled at the last minute.
Downing Street conceded some payments had been made to Kigali, but would not set out how much or when under the “confidential” deal signed two months ago.
The payments came despite no one-way flights for migrants who arrive in the UK through unauthorised journeys taking off.
Pressed whether any of it had already been spent, she said: “Part of it because we needed to get ready and we were ready to receive the first migrants on the 14th.”
The first flight was halted after an interim injunction from the European Court of Human Rights, pending a decision on the legality of the scheme in the UK courts.
Mr Johnson’s spokesman said: “The agreement with Rwanda with regards to the funding is confidential. It’s fair to say we have made some payments as part of that migration partnership, that’s correct.
“Obviously there’s a number of pieces of work they need to do to get things ready, to get things set up.”