Whilst there is no legal requirement to pay relocation allowances it is highly likely that your employer will have a formal policy on such allowances. In organisations that recognise trade unions such a policy may have been negotiated with your union representatives.
Identify what the agreed relocation allowance should be. If you have not been paid the correct amount raise the issue with your manager. If you are member of a trade union consult your union representative.
If the matter can't be resolved by these informal means you may have to raise a formal grievance using your organisation's formal grievance procedure.
In some circumstances a policy or formal agreement on relocation allowances might become incorporated into your individual contract of employment. If this is the case and you have failed to achieve a satisfactory outcome, by pursuing your organisation's grievance procedure and you believe that you can establish that you have not been paid the correct amount of allowance, you could consider whether it's worth bringing a claim for breach of contract. Seek advice before going down this route.
Just One More Click: Doggy drama about staying safe online at work.