The necessary journal entries on January 15 are:
Reinstatement of the account receivable:
\[
\begin{array}{c}
\text{Debit: Accounts Receivable} \quad \$600 \\
\text{Credit: Bad Debt Expense} \quad \$600 \\
\end{array}
\]
Receipt of cash:
\[
\begin{array}{c}
\text{Debit: Cash} \quad \$600 \\
\text{Credit: Accounts Receivable} \quad \$600 \\
\end{array}
\]
\(\boxed{\text{Journal entries recorded successfully}}\)