You wouldn’t be able to have a dictionary-like object here. But if an array of each item as key-value pair works for you. You can do that by
"paymentObject": {
"bsonType": "array",
"items": {
"bsonType": "object",
"properties": {
"key": {
"bsonType": "string"
},
"value": {
"bsonType": "string"
}
}
}
}