Let us start with the $2000 that came in June. We know that the shop was closed in May. This means that the $2000 came from the month of April.
Only 1/4 of the money is paid two months later. This means that the money made in April must have been = 2000 x 4 = $8000
We know that the second 1/4 is paid the next month. This means that out of the $8000, 1/4 is paid in the next month (i.e. May). 1/4 x 8000 = $2000
$2000 out of $3000 that came in May was made in April. Since the shop was closed in May, the remaining $1000 must have come from March.
Total sale in March = $1000 x 4 = $4000
We know that 1/2 of the money is paid immediately.
So out of the $4000 that was made in March, $2000 was made immediately. The remaining $2000 must have come from January and February.