Look at the legal issues involved here e.g
Acceptance must be communicated to the offeree (cite case relevant to general rule - Powell v Lee)
Exception:It is assumed that all businesses take part in 'good business practices' whereby the communication of acceptance should have been communicated. (case law - Brinkibon v Stahag Stahl (1982))
Look at whether Mandy was making a counter offer when she asked about paying after december or whether this was merely and inquiry of information/a small change in the terms(case law for counter offer: Hyde v Wrench, case law for invitation to treat/negotiation Harvey v Facey)
Look at whether it is important that Mandy arranged loans in order that she could pay Bella (case law - Brogden v Metropolitan Railway - conduct can constitute as acceptance)
Once you have argued/reasoned these issues through, decide whether there was a contract and whether there was a breach of contract by Bella.