you can leave the correct amount of oil in the engine 😉
Nothing bad happened to the oil.
Even the oil that you have drained could be re-used.
There is a sight glass in the clutch cover. When the bike is upright (not on side stand!!!) correct=upper oil level is marked with a line on the cover at the upper end of that glass.
Low level is marked with a line at the lower end of the glass.
Difference from upper to lower level is approx. 1 liter.
Total amount of oil in the engine should be around 4 liters.
… and don’t forget to kick the mechanic’s back and get some money back, if he had charged you for more than 4 liters of oil !