“Start doing what is necessary, then do what is possible, and suddenly you are doing the impossible” is attributed to St. Francis in thousands of web pages. But books.google.com can't find it in any book. scholar.google.com finds the first reference in a book by John C. Maxwell in 2011.
I think if Francis said anything similar, it would have been in print before 2000. But if he said it, he would have said it in Italian or French, so a pre-2000 translation might not have that exact phrasing.
UPDATE: Google found one instance before 1970, on a web page—rather Google thinks it was done in January of 1970! Examination of the page finds a mention of someone dying in 2018, and the quote attributed to Clare of Assisi.
Seems that we are not going to get a clear answer from Google, but it does seem that someone made it up not long before 1987 and that it become misattributed to Francis (and Clare).