Your actual ITC won't change based on these things.
Your actual ITC will be 100
However, coming to the part eligibility of ITC, provided that you had shown Rs. 100 ITC in GSTR 3B, then you can claim Rs. 100 as your ITC irrespective of the fact that one of your supplier has not shown the bill in his GSTR 1 as the difference amount is less than 20%. Maximum difference that can be allowed in GST is 20% limit for the period prior to 01st january 2020. However, it is advisable for you to ask the other party to file GSTR-1 to avoid future complications.
Alternatively, you can pay back the government Rs. 20 with interest and recover the same from the client
Coming to the books of accounts part... If you can't modify the books of accounts of year 18-19, then pass rectification entries in current year(Pass those entries in current year with a narration that these are prior period items and not shown in prior period)