John Gill Commentary Genesis 43:4

"If thou wilt send our brother with us, we will go down and buy thee food:" — Genesis 43:4 (ASV)

If you will send our brother with us
Give orders for his going with us, and put him under our care: we will go down and buy you food ;
signifying, on the above condition, that they were ready and willing to take a journey into Egypt, and buy provisions for him and his family, otherwise not.