db={"Products":[{"_id":ObjectId("5e4633eaa7095f26d44a43c3"),"name":"Acrylic Frame Magnetic","units":"50","fbasku":"AFM-CL-0507-FBA","upc":"642709233954"},{"_id":ObjectId("5e4633eaa7095f26d44a43c4"),"name":"Apron Polka Dot","units":"488","fbasku":"APD-RD-03PC-FBA","upc":"642709233961"},{"_id":ObjectId("5e4633eaa7095f26d44a43c5"),"name":"Acrylic Sign Holder - 5x7","units":"632","fbasku":"ASH-GD-0507-FBA","upc":"642709233978"}],"Transactions":[{"_id":ObjectId("5e44e8ed8f4cd40bd09d5ce8"),"type":"adjust","fbasku":"AFM-CL-0507-FBA","units":25.0,"comment":"example transaction"},{"_id":ObjectId("5e4c9ab642c5a232042f4b67"),"type":"send","fbasku":"AFM-CL-0507-FBA","units":75.0,"comment":""},{"_id":ObjectId("5e4c9ab642c5a232042f4b69"),"type":"send","fbasku":"AFM-CL-0507-FBA","units":5.0,"comment":""}]}

db.Products.aggregate([{$lookup:{from:"Transactions",let:{fbasku:"$fbasku"},pipeline:[{$match:{$expr:{$eq:["$fbasku","$$fbasku"]}}},{$project:{units:1,_id:0}}],as:"transaction_docs"}},{$project:{_id:0,fbasku:1,units:{$add:[{$toInt:"$units"},{$reduce:{input:"$transaction_docs",initialValue:0,in:{$add:["$$value","$$this.units"]}}}]}}}])